Progressively higher resolution NMR measurement demands higher signal to noise performance and a baseline free-of artifact and distortion. A known methodology for obtaining improved signal to noise performance involves the sampling of time domain waveforms at a rate in excess of twice the rate corresponding to the highest frequency component of interest in the waveform. This higher than required (oversampling) rate .omega..sub.s increases the spectral width proportionately and causes the uncorrellated noise, or noise from broadband sources to be spread over a wider bandwidth. Only a relatively narrow portion of this expanded bandwidth contains the data of interest. Were the expanded bandwidth to be directly accomodated, the requirement for both memory and time for effecting the Fourier transformation would become impractical or prohibitive. Alternately, and preferably, the oversampled data is subject to a digital filter which returns a single datum from a plurality of oversampled data through convolution of the oversampled data with a selected filter function. Digital filter theory and practice are well known to the artisan.
In the prior art, the use of digital filters commonly introduces artifact and/or distortion to the spectral baseline. An origin for this effect is recognized in the time delays associated with filters which operate upon the time domain waveform. For example, a digital filter fuctions initially without the requisite history of oversampled data upon which to operate. U.S. Pat. No. 5,652,518, commonly assigned and incorporated herein by reference, treats this initial lack of data with use of pseudodata derived from the filter function to achieve a real-time digital filter exhibiting reduced baseline distortion.
It is known in the prior art to reduce baseline distortion arising from the transient ringing response of the filter. The analog filter also exhibits a finite response time. In the prior art baseline distortion was reduced by arranging the time of acquisition of data (specifically, the second datum acquired) in the vicinity of such transient response so as to avoid the transient filter response and acquire all data commencing with the second point on the flat (unity gain) portion of the filter characteristic, and by commencing data acquisition with the first sampling point representing the true time origin of the impulse, which has, in fact, been delayed by the receiver and analog filters therein.